chore: ensure all exported files have a consistent filename

CHANGELOG.md

@@ -7,6 +7,8 @@
 ### Chore & Maintenance
 
 - `[*]` [**BREAKING**] Drop support for Node v10 and v15 and target first LTS `16.13.0` ([#12220](https://github.com/facebook/jest/pull/12220))
+- `[@jest/core]` Use `index.ts` instead of `jest.ts` as main export ([#12329](https://github.com/facebook/jest/pull/12329))
+- `[jest]` Use `index.ts` instead of `jest.ts` as main export ([#12329](https://github.com/facebook/jest/pull/12329))
 
 ### Performance
 

packages/jest-core/package.json

@@ -2,12 +2,12 @@
   "name": "@jest/core",
   "description": "Delightful JavaScript Testing.",
   "version": "27.5.1",
-  "main": "./build/jest.js",
-  "types": "./build/jest.d.ts",
+  "main": "./build/index.js",
+  "types": "./build/index.d.ts",
   "exports": {
     ".": {
-      "types": "./build/jest.d.ts",
-      "default": "./build/jest.js"
+      "types": "./build/index.d.ts",
+      "default": "./build/index.js"
     },
     "./package.json": "./package.json"
   },

packages/jest/package.json

@@ -2,12 +2,12 @@
   "name": "jest",
   "description": "Delightful JavaScript Testing.",
   "version": "27.5.1",
-  "main": "./build/jest.js",
-  "types": "./build/jest.d.ts",
+  "main": "./build/index.js",
+  "types": "./build/index.d.ts",
   "exports": {
     ".": {
-      "types": "./build/jest.d.ts",
-      "default": "./build/jest.js"
+      "types": "./build/index.d.ts",
+      "default": "./build/index.js"
     },
     "./package.json": "./package.json",
     "./bin/jest": "./bin/jest.js"

scripts/buildUtils.js

@@ -74,6 +74,25 @@ module.exports.getPackages = function getPackages() {
       `Package "${pkg.name}" does not export correct files`,
     );
 
+    if (pkg.types) {
+      assert.strictEqual(
+        pkg.main,
+        './build/index.js',
+        `Package "${pkg.name}" should have "./build/index.js" as main`,
+      );
+      assert.strictEqual(
+        pkg.types,
+        './build/index.d.ts',
+        `Package "${pkg.name}" should have "./build/index.d.ts" as types`,
+      );
+    } else {
+      assert.strictEqual(
+        pkg.main,
+        './index.js',
+        `Package "${pkg.name}" should have "./index.js" as main`,
+      );
+    }
+
     if (pkg.bin) {
       Object.entries(pkg.bin).forEach(([binName, binPath]) => {
         const fullBinPath = path.resolve(packageDir, binPath);